Milkov(2004)による〔『Global estimates of hydrate-bound gas in marine sediments: how much is really out there?』(184,185,193p)から〕

 It is generally assumed that oceanic gas hydrates contain a huge volume of natural gases, mainly methane. The most widely cited estimate of global hydrate-bound gas is 21×15 m3 of methane at STP (or 〜10,000 Gt of methane carbon), which is proposed as a “consensus value” from several independent estimations. This large gas hydrate reservoir is further suggested as an important component of the global carbon cycle and as a future energy source. Here, I present a revised and updated set of well-justified global estimates and discuss how and why they changed over time. It appears that the global estimates of hydrate-bound magnitude from 1970s−early 1980s (estimates on the order of 1017−1018 m3) to late 1980s−early 1990s (1016 m3) to late 1990s−present (1014−1015 m3). The decrease of estimates is a result of growing knowledge of the distribution and concentration of gas hydrates in marine sediments and ongoing efforts to better constrain the volume of hydrate-bearing sediments and their gas yield. These parameters appear to be relatively well constrained at present through DSDP/ODP drilling and direct measurements of gas concentrations in sediments. The global estimate of hydrate-bound gas that best reflects the current knowledge of submarine gas hydrate is in the range (1−5)×1015 m3 (〜500−2500 Gt of methane carbon). A significantly smaller global gas hydrate inventory implies that the role of gas hydrates in the global carbon cycle may not be as significant as speculated previously. Gas hydrate may be considered a future energy source not because the global volume of hydrate-bound gas is large, but because some individual gas hydrate accumulations may contain significant and concentrated resources that may be profitably recovered in the future.

Keywords: Gas hydrate; Methane; Global estimates; Carbon cycle; Energy source』

1. Introduction

表1 海底ガスハイドレート中のメタンの世界における見積り
(×1015m3)(Kvenvolden, 1999による)
最低値−最高値 最もよい見積り 文 献
5−25   Trofimuk et al.(1977)
  7600 Dobrynin et al.(1981)
  3.1 McIver(1981)
  40 Kvenvolden and Claypool(1988)
  10 Makogon(1981)
  21 Kvenvolden(1988)
  21 MacDonald(1990)
26−140 26 Gornitz and Fung(1994)
23−91 46 Harvey and Huang(1995)
  1 Ginsburg and Soloviev(1995)
  7 Holbrook et al.(1996)
  15 Makogon(1997)
2−20   Dickens et al.(1997)

Kvenvolden,K.A.(1999): Potential effects of gas hydrate on human welfare. Proceedings of National Academy of Science, 96, 3420-3426.』

2. Review of global estimates in a chronological order

表2 海底ガスハイドレート中のメタンの世界における見積りの改定値



文 献
30213085 3053 Trofimuk et al.(1973)
  1135 Trofimuk et al.(1975)
  1573 Cherskiy and Tsarev(1977)
  〜1550 Nesterov and Salmanov(1981)
  >0.016 Trofimuk et al.(1977)
110130 120 Trofimuk et al.(1979)
  3.1 MIver(1981)
5−25 15 Makogon(1981), Trofimuk et al.(1981,1983a)
  15 Trofimuc et al.(1983b)
  40 Kvenvolden and Claypool(1988)
  20 Kvenvolden(1988)
  20 MacDonald(1990)
26.4139.1 26.4 Gornitz and Fung(1994)
22.790.7 45.4 Harvey and Huang(1995)
  1 Ginsburg and Soloviev(1995)
  6.8 Holbrook et al.(1996)
  15 Makogon(1997)
  0.2 Soloviev(2002)
35 4 Milkov et al.(2003)
15 2.5 本研究

3. Discussion
 3.1. Decrease of global estimates as a function of time and the growing understanding of natural gas hydrate
 3.2. How much is really out there?
4. Implications
 4.1. Gas hydrate as a component of the global carbon cycle

図4 世界のハイドレート量の見積りが異なる場合の、地球の有機炭素の分布(ケロジェンおよびビチューメンのような分散した有機炭素は除く)。値は炭素Gt(ギガトン)。
(a) ガスハイドレート中のメタン炭素の見積りを10000Gtとした場合の分布(Kvenvolden, 1993)。
(b) 世界のハイドレートに固定されたガスの体積が上限値であると仮定して、世界のガスハイドレートの見積りを改定した場合の分布。
(c) 世界のハイドレートに固定されたガスの体積が下限値であると仮定して、世界のガスハイドレート量の見積りを改定した場合の分布。』

 4.2. Gas hydrate as a potential energy source
5. Conclusions